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Key House races to watch in 2018

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Key House races to watch in 201801:41

(CNN)The 2018 midterm elections aren't for another 271 days. But, the shape of the playing field on which the battle for control of the House will be fought is beginning to form -- and it looks very, very problematic for Republicans.

On Thursday, the Cook Political Report, a leading non-partisan handicapping site, moved 21 House districts in favor of Democrats; Cook House editor David Wasserman suggests that still might underestimate Democrats' ceiling in terms of seat gains this fall.

"Most new district-by-district fundraising and polling numbers are downright terrible for Republicans, even in seats previously thought to be safe," writes Wasserman.

Those numbers have to be scary for Republicans hoping to cling to their 24-seat majority. Why? Because what we've seen of late -- fueled by remarkably successful fundraising by dozens of Democratic candidates -- is an expansion of the ground where this election will be fought. And that expansion is entirely in Republican territory.

What that means in practical terms is that Democrats have more margin for error as they try to net two dozen seats. If a race they expected to be competitive suddenly collapses, it doesn't doom their chances as they now have 2, 3 or even 4 other seats to replace it.

The other potential impact of the widening of the playing field in Democrats' favor is that if there is a massive wave that sweeps the country -- and signs suggest something is building -- then the party could score massive gains as any even quasi-marginal seat falls to them.

(That, by the by, is what happened for Republicans in 2010 when they netted 63 seats including in some districts that not even the most optimistic Republican operative thought they had a chance to win.)

The point here is this: The election may feel like it is a long way away. But, the map on which Democrats and Republicans will fight is evolving in important ways as I type. And that evolution is all in favor of Democrats.